Thin Solid Films 428 (2003) 201–205 0040-6090/03/$ - see front matter  2002 Elsevier Science B.V. All rights reserved. PII:S0040-6090 Ž 02 . 01205-1 Thickness dependence of photoemission and X-ray fluorescence spectra in epitaxial NiO layers on Ag(100) S.A. Krasnikov *, A.B. Preobrajenski , T. Chasse , R. Szargan a, a a,b a ´ Wilhelm-Ostwald-Institut fur Physikalische und Theoretische Chemie, Universitat Leipzig, Linnestr. 2, D-04103 Leipzig, Germany a ¨ ¨ ´ Institut fur Oberflachenmodifizierung, Permoserstr. 15, D-04318 Leipzig, Germany b ¨ ¨ Abstract NiO films of different thickness (0.8–12 ML) were grown in situ on Ag(100) and studied by low-energy electron diffraction, X-ray photoemission, X-ray photoelectron diffraction and X-ray fluorescence spectroscopy. The photoemission and X-ray fluorescence spectra recorded from epitaxial NiO(100) films provide evidence for the sensitivity of the inner-shell electronic transitions to the geometry of the first and the second coordination spheres around the absorbing Ni ion. The relative intensities 2q of individual structures in the Ni 2p photoemission spectra change significantly with increasing thickness of the film. They are supposed to be influenced by competition between transitions of screening electrons coming either from the surrounding ligands or from ligands around a neighboring transition metal ion. These findings are consistent with the results of theoretical studies of non-local screening effects explaining the shape of the main line in the Ni 2p spectra. Information about local partial densities of states was obtained from a comparative analysis of Ni L and O K X-ray fluorescence and valence-band photoemission spectra.  2002 Elsevier Science B.V. All rights reserved. Keywords: Nickel oxide; Silver; Epitaxy; Photoelectron diffraction; X-Ray photoemission; X-Ray fluorescence 1. Introduction The investigation of the electronic structure of tran- sition metal (TM) oxides has been an active field of research for several decades, arising from many puzzling issues, such as the nature and origin of the insulating state in partially filled 3d systems and the existence of metal–insulator transitions w1–3x. In recent years, ultra- thin films and multi-layers have become very popular because of their interesting and novel physical proper- ties. One of the interesting aspects of the electronic structure of thin NiO films is the influence of the film thickness on the antiferromagnetic order. Indeed, a strong thickness dependence of magnetic moments has been found in ultrathin epitaxial NiO films studied by X-ray magnetic linear dichroism in Ni L absorption 2,3 w4x. In particular, even for 20-ML-thick films the Neel ´ temperature was reported to be strongly reduced from the bulk value, and 5-ML-thick layers were even sup- posed to be paramagnetic at room temperature. Since the crystal quality and morphology of ultrathin films *Corresponding author. Tel.: q49-341-9736457; fax: q49-341- 9736399. E-mail address: krasnikov@rz.uni-leipzig.de (S.A. Krasnikov). can strongly influence their electronic and magnetic properties, suitable substrates are necessary to ensure high-quality epitaxial growth. For the formation of epitaxial NiO films, silver substrates with different orientation were found to be appropriate due to the small lattice mismatch (;2%) and chemical inertness of the latter w5,6x. The electronic structure of thin NiO layers can be probed by X-ray photoemission spectroscopy (XPS) and X-ray fluorescence spectroscopy (XF). In TM oxides, where the interaction between the core hole and the valence electrons is strong, core-level XPS exhibits characteristic features, which cannot be completely inter- preted using the final state description in terms of excited configurations of a TM–O cluster w7x. Although the satellite structure in Ni 2p XPS of NiO (at approx. 1.5 eV higher binding energy with respect to the main peak) has been known for decades, its origin is still open for discussion w8x. The valence band (VB) of the NiO films can be probed, in addition to VB XPS, by X-ray fluorescence due to its elemental selectivity and dipole selection rules, which allow study of the Ni 3d component of the VB separately from other states. Nowadays XF is increasingly popular due to the advent